The present invention relates to an exhaust gas purification apparatus for making harmless hydrocarbon (HC) which is discharged in relatively large amounts at cold-starting of an internal combustion engine such as a gasoline engine.
As a prior art, an exhaust gas purification system is known, wherein a zeolite HC adsorbent is installed in part of an exhaust pipe to trap HC as an unburnt component which is discharged at cold-starting of an internal combustion engine, and HC separated from the adsorbent is purified by an oxidation catalyst installed downstream from the adsorbent. (By way of example, refer to Japanese Patent Unexamined Publication No. 2-135126.)
In a conventional system, since a HC adsorbent is placed over an entire section across the exhaust passage upstream from a catalyst so that all exhaust gas passes through the HC adsorbent, a problem arises in that an exhaust resistance is made high by the HC adsorbent, whereby there is a fear that an output of an internal combustion engine is reduced. Also, since all exhaust gas passes through the HC adsorbent, the heat of the exhaust gas is transferred to the adsorbent and the temperature increase of the adsorbent is accelerated. As a result, there is also a fear that HC adsorbed by the adsorbent begins to desorb out before the downstream oxidation catalyst is heated to an activation temperature at which it starts purifying exhaust gas such as oxidation of HC and HC is discharged while still unpurified. In order to prevent this disadvantage, needs are resulted such as addition of an auxiliary heating means for rapidly increasing the temperature of the oxidation catalyst to activate. Accordingly, the system becomes complex and costly.
Further, another problem arises in that, generally, the zeolite HC adsorbent must be placed relatively downstream in the exhaust pipe where the temperature of the exhaust gas becomes low because the heat resistance of the zeolite HC adsorbent is about 700.degree. C. which is low and, thus, as to the oxidation catalyst which is provided more downstream where the exhaust gas temperature is lower, a burden is put on the auxiliary heating means to activate the oxidation catalyst early, for instance, extremely large amounts of electric power are consumed if the auxiliary heating means is an electric heater.
Accordingly, it is an object of the invention to provide an exhaust gas purification apparatus or, more particularly, and apparatus for reducing HC at engine start, wherein engine output is not reduced by exhaust resistance due to an adsorbent, there is no need to provide any special auxiliary heating means such as an electric heater in a catalyst for purifying the HC desorbed from the adsorbent, and a low heat-resistant adsorbent like zeolite can be used.
The invention provides an exhaust gas purification apparatus in which a bent or branched portion is formed in part of an exhaust passage connecting exhaust ports of an internal combustion engine to a catalyst for purifying at least HC from the exhaust gas, and a HC adsorbent is placed in an extended portion forward with respect to a blow-down direction of the exhaust gas which travels straight without turning at the bent portion.
In a second embodiment, the HC adsorbent is adapted to adsorb HC in the exhaust gas in a temperature range in which the catalyst is inactive, and to desorb the adsorbed HC mainly in a temperature range in which the catalyst is active.
Similarly, in a third embodiment, the HC adsorbent carries a zeolite HC adsorbent by means of a heat-resistant carrier.
In a fourth embodiment, a rectifier means for guiding the exhaust gas is provided between the exhaust ports and the HC adsorbent.
In a fifth embodiment, the HC adsorbent includes a cooling means for delaying temperature rise of the HC adsorbent.
In a sixth embodiment, the HC adsorbing means has a cover means for delaying temperature rise of the HC adsorbent.
In a seventh embodiment, an EGR passage to be connected to an intake passage of the internal combustion engine is connected downstream from the HC adsorbent, and an EGR control valve for opening/closing the EGR passage is provided in part of the EGR passage.
Similarly, in an eighth embodiment, an air-fuel ratio sensor is provided upstream from the catalyst, and control cycle of air-fuel ratio feedback control of the internal combustion engine based on a detected value of the air-fuel ratio sensor is made shorter upon opening of the EGR control valve than that upon closing of the EGR control valve.
In a ninth embodiment, the HC adsorbent is placed in a junction in an exhaust manifold.
Similarly, in a tenth embodiment, the HC adsorbent is formed with a communication path for communication among spaces existing forward with respect to the blow-down direction of the exhaust gas flowing from each exhaust port of a plurality of cylinders having different exhaust timing in the internal combustion engine.
In an eleventh embodiment, throat for controlling the flow is provided in the communication path.
In a twelfth embodiment, HC purification catalyst is carried by a portion of the HC adsorbent which is heated to a relatively high temperature.
Similarly, in a thirteenth embodiment, the HC adsorbent comprises a plurality of materials having different specific heat, and the HC purification catalyst is carried by a carrier made of a material having a small specific heat.
In a fourteenth embodiment, a carrier of the adsorbent is made of a ceramic, and the carrier of the HC purification catalyst is made of a heat-resistant metal such as stainless steel.
The hydrocarbon (HC) discharged in relatively large amounts as an unburnt component of a fuel, for instance, at engine start includes particles of various sizes from large particles such as liquid fuel particles to small particles such as vaporized gaseous molecules, and, more particularly, at engine start, includes many particles of relatively large particles sizes and large specific gravity like liquid fuel particles because the temperature in the combustion chamber is still low.
Since each of the embodiments of the invention commonly comprises the basic construction shown in the first embodiment, when an exhaust gas containing many relatively large HC particles passes through the bent portion formed in part of the exhaust passage at engine start, the gaseous components of the exhaust gas smoothly curve and flow toward the catalyst substantially without receiving any resistance because of their small specific gravity, but many of the HC particles of large specific gravity separate from the exhaust gas by the action of inertia and travel straight in the blow-down direction along with a relatively small amount of exhaust gas, and strike against HC adsorbent provided forward, where they absorbed and trapped.
Accordingly, when the catalyst is still inactive at engine start, the HC-rich exhaust gas is prevented from passing through the catalyst and being discharged outside. While HC is adsorbed by the adsorbent, the catalyst receives the heat of the exhaust gas to increase in temperature, and reaches the activation temperature to begin purifying the HC. Since, in each embodiment of the invention, most of the exhaust gas flows while curving at the bent portion formed in part of the exhaust passage, an amount of the exhaust gas flowing into the HC adsorbent is slight, and thus no hindrance even if a relatively low heat-resistant adsorbent, for instance, a zeolite, is provided relatively upstream in the exhaust passage along with the bent portion, whereby the catalyst can be provided relatively upstream in the exhaust passage.
Accordingly, as compared with the conventional case wherein the catalyst is provided downstream from the HC adsorbent provided downstream in the exhaust passage where the temperature of the exhaust gas becomes low, the temperature rise (activation) of the catalyst becomes faster. In addition, the amount of exhaust gas flowing into the HC adsorbent is all in the prior art, whereas, in each embodiment of the invention, it is only partial and thus the temperature rise of the HC adsorbent is delayed, so that the start of desorption of the adsorbed HC is generally delayed. In consequence, the catalyst can fully increase in temperature during that time without relying on an auxiliary heating means.
However, since the HC adsorbent gradually increases in temperature in absorbing the heat of the exhaust gas little by little, the amount of HC adsorbed by the adsorbent is eventually exceeded by that of the HC desorbed from the adsorbent but, at this point, the catalyst has already keen activated by increasing temperature through the absorption of heat of the exhaust gas, so that the desorbed HC is purified by the catalyst and is not discharged outside. In addition, since most of the exhaust gas does not pass through the HC adsorbent, there is no possibility of reduced engine output due to increased exhaust resistance.
In the second embodiment of the invention, the HC adsorbent is adapted to adsorb HC in the exhaust gas in a temperature range where the catalyst is inactive, and to desorb the adsorbed HC mainly in the activation temperature range of the catalyst and, as is apparent from the above description, since the HC adsorbent is adsorbing the HC in the exhaust gas after engine startup and before the catalyst reaches the activation temperature, HC can be prevented from passing through the inactive catalyst and being released into the atmosphere still not treated. Accordingly, a time margin can be obtained for causing the catalyst temperature to rise, thereby to activate the catalyst while the HC adsorbent is adsorbing HC.
Similarly, in the third embodiment, the HC adsorbent is made by causing a heat-resistant carrier to carry a zeolite adsorbent, but in every embodiments of the exhaust gas purification apparatus of the invention, the HC adsorbent is of a construction in which it is relatively difficult to absorb the heat of the exhaust gas as described above, and, accordingly, no heat resistance problem occurs if a zeolite adsorbent of a relatively low heat resistance is used, whereby it is possible to take full advantage of the excellent HC adsorption performance of the zeolite HC adsorbent.
In the fourth embodiment, since there is provided a rectifier means for exhaust gas of various shapes between the exhaust ports and the HC adsorbent, the portion of exhaust gas of large specific gravity containing many HC particles is guided by the rectifier means to reach the HC adsorbent, and HC particles can be efficiently trapped by the HC adsorbent.
In the fifth embodiment, since a construction is given for positively cooling the HC adsorbent by the use of a cooling means, the temperature rise of the HC adsorbent becomes more gradual and the timing of desorption of the adsorbed HC from the HC adsorbent is delayed, so that the time margin for allowing the catalyst to increase in temperature becomes larger accordingly. Consequently, an auxiliary heating means for rapidly activate the catalyst is unnecessary, and the power consumed by such means can be saved.
In the sixth embodiment, since a cover means is provided to cover the HC adsorbent for preventing the exhaust gas from directly striking the HC adsorbent, the heat of the exhaust gas is difficult to be absorbed by the HC adsorbent, by which an effect similar to the fifth embodiment can be obtained.
In the seventh embodiment, an EGR passage is connected downstream from the HC adsorbent and an EGR control valve for opening/closing the EGR passage is provided in part of the EGR passage, and, normally, the EGR control valve is closed because EGR is not performed when the engine starts up and there is no exhaust gas flow passing through the EGR passage, so the HC adsorbed on the HC adsorbent is temporarily held by the adsorbent.
Then, engine warmup proceeds and EGR begins with the lapse of time and, when the EGR control valve opens to cause an exhaust gas flow in the EGR passage, the adsorbent is heated by the exhaust gas and the HC adsorbed on the adsorbent starts to desorb therefrom. The HC having desorbed from the adsorbent flows into the intake passage along with the exhaust gas (or EGR gas) passing through the EGR passage, mixes with the fuel mixture, and is burnt in the combustion chamber of the engine, whereby it is treated.
In addition the exhaust gas temperature increases for a high load, but, normally, EGR is not performed at this stage, and thus the EGR control valve closes to prevent the exhaust gas from passing through the EGR passage, whereby the adsorbent temperature is kept relatively low. Accordingly, it is completely possible to use an adsorbent, such as zeolite, having a relatively low heat-resistance.
The air-fuel ratio of the engine varies when the EGR control valve opens and the HC having desorbed from the adsorbent is purged into the intake passage of the engine, but, in the eighth means, the air-fuel ratio of the engine is controlled through feedback by an output signal of an air-fuel ratio sensor provided in the exhaust passage or EGR passage upstream from the adsorbent, for instance, the control cycle of the air-fuel ratio of the engine is varied in synchronized with the opening/closing control of EGR control valve, so that the large variation of the air-fuel ratio can be prevented and no problem arises.
In the ninth embodiment, since the HC adsorbent is placed in the junction of the exhaust manifold, the HC adsorbent can efficiently trap the HC in the exhaust gas and no special container need be provided for the HC adsorbent.
In the tenth embodiment, further developed from the ninth embodiment, wherein a communication path in the HC adsorbent allows communication among spaces forward with respect to the blow-down direction of the exhaust gas flowing from each exhaust port of a plurality of cylinders having different exhaust timing in the internal combustion engine, the pressure at one of the two ends of the communication path is higher than that at the other one because the exhaust timing of the plurality of cylinders differs, and thus the pressure difference causes an exhaust gas flow passing through the communication path while reversing, whereby the HC is efficiently absorbed when it passes through the communication path in the HC adsorbent.
In the eleventh embodiment, since a throat is provided in the communication path described above, an amount of exhaust gas passing through the communication path can be adjusted by appropriately setting the throat, thereby for controlling the amount of HC to be adsorbed and the degree of temperature rise.
Further, in the twelfth embodiment of the invention, since the HC purification catalyst is carried in HC adsorbent heated to a relatively high temperature, the HC desorbed from the HC adsorbent can be immediately purified by the catalyst. In addition, the catalyst is carried in the HC adsorbent which is heated to a relatively high temperature, so that it is quickly activated and can purify the HC at an early stage.
In the thirteenth embodiment, since the HC adsorbent is comprised of a plurality of materials having different specific heat and the HC purification catalyst is carried in a carrier made of a material of small specific heat, the catalyst is also activated and purifies the HC at an early stage because the material, having a small specific heat, increases in temperature relatively quickly.
In the fourteenth embodiment, as a specific example of the plurality of materials having different specific heat in the thirteenth embodiment, the carrier of the adsorbent is a ceramic and the carrier of the HC purification catalyst is a heat-resistant metal such as stainless steel. The typical action as described above is thus produced.